Chemistry - Gas Laws Test Study Guide

We can get real gases to act closely to ideal behavior by manipulating the variables is such a way as to make the gas particles behave as the postulates suggest.

The conditions for real gases to behave like ideal gases are HIGH TEMP and LOW PRESSURE.

What units are (could be) used for temperature and volume while studying the relationship between the variables? (Circle all that apply) a. K and L b. K and mL c. ̊C and L d. ̊C and mL Explain the reasoning used for the choice(s) above.

a; Temp must be in K, and for gases, volume is measured in L.

Ideal gases perfectly follow the ideas of the KMT (kinetic molecular theory). These ideas, or postulates, are as follows:

1. Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion. 2. These particles move in a straight line until they collide with another particle or the walls of the container. 3. These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space. 4. There is no force of attraction between gas particles or between the particles and the walls of the container. 5. Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container. 6. The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else.

Kinetic Molecular Theory of Gases

A sees of ideas used to account for the behavior of ideal gases. The theory describes gas as volumeless particles in constant, random motion that exhibit no intermolecular attractions and undergo completely elastic collisions with each other and with the walls of the container.

Avogadro's Law

At STP (1.00 atm, O ̊C), 1.00mol of any gas takes up 22.4L

At what conditions do real gases behave like ideal gases? Explain why this is so?

At high temperatures and low pressures. Low P allows the gas particles to move farther apart - reducing the significance of their volume and reducing any chance for intermolecular forces of attraction to develop. High temps mean that the particles are moving quickly, so even if particles did get close, forces of attraction do not have time to develop.

What are the 4 assumptions of the KMT?

Constant random motion, collisions are perfectly elastic, gases have no appreciable volume, pressure is about collisions

Suppose you had 20 large particles in the simulation and then you changed it to 20 small particles. If the temperature and volume were kept the same, the pressure exerted by 20 large particles would be _______ the pressure exerted by the small particles.

Equal to

Expandability vs Diffusion

Expandability. Just as gases are compressible, they are also expandable. Diffusion causes them to expand to take the shape of any container in which they are placed, filling the container's volume. Liquids are much less compressible and solids even less so.

If the volume of a gas is held constant, what effect would decreasing the pressure have on the temperature?

Gay-Lussac's law. If we decrease the pressure, the temperature would have to drop in order for the volume to stay the same.

What are the assumptions of the kinetic molecular theory? (How do gases behave?)

I. Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion. II. These particles move in a straight line until they collide with another particle or the walls of the container. III. These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space. IV. There is no force of attraction between gas particles or between the particles and the walls of the container. V. Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container. VI. The average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else.

Most all, if not every, metal aerosol can has a warning label indicating to its user to keep away from heat sources and open flames. Explain the need for the warning and the danger that could result from ignoring it in terms of the kinetic molecular theory.

If the particles inside a closed container heat up, they will collide for frequently and harder with the inside of the container, causing a rise in pressure. If the pressure were to get high enough the can would explode.

Why these conditions?

If you have low pressure the particles can get farther apart, making postulate 3 more true. Both the high temp and the low pressure help to make postulate 4 more true. If the particles are far apart it will be hard for them to attract on another. Also, if the particles are moving really fast (high T), even if they get close enough to attract one another they are only going to be near one another for a moment of time, too short to have any significant attraction.

Explain in terms of the kinetic theory what happens to the pressure inside of a closed container and what happens to the volume of a gas as your increase the temperature.

If you increase the temperature of a gas in a closed container the particles move faster, causing more frequent and harder collisions, which caused the pressure to rise. If you increase the temperature of a gas the volume will get larger. The particles will move faster in their random motion which causes the particles to more quickly spread out and occupy more space.

Describe what would happen if you put a balloon in boiling water. Explain in terms of the KMT why this would happen.

If you put a room temperature balloon in boiling water the temp of the gas inside the balloon would rise causing the particles to move faster. This, in turn, would cause the particles to hit the inside of the balloon more frequently and harder. This will cause the balloon to expand.

Explain how decreasing the pressure on a sample of gas affects the volume if the temperature is held constant. Be sure to include the behavior of the gas.

It will cause the volume to increase. If the temp is the constant, the particles must travel with the same average speed. To decrease the pressure when the particles are not slowing down, we will have to make the volume larger.

Explain how decreasing the temperature on a sample of gas affects the pressure if the volume is held constant. Be sure to include the behavior of the gas particles.

It will decrease the pressure. Pressure is about frequency and force of the collisions of the gas particles with a surface. If you decrease the temperature of a gas, its particles will be moving slower. This causes the frequency of collisions to decrease as it takes the gas longer to get from side to side. It will also decrease the force of the collision.

Explain how increasing the volume on a sample of gas affects the temperature if the pressure is held constant. Be sure to include the behavior of the gas.

It will increase the temperature. To keep the pressure constant we will need to keep collision frequency and force (together) the same. To do this, with a larger volume, the particles will need to speed up - thus, the temperature must increase.

inelastic collision

Objects stick together, momentum is conserved, but some kinetic energy is lost. a type of collision in which the kinetic energy after the collision is less than the kinetic energy before the collision. A collision in which the colliding objects become distorted, generate heat, and possibly stick together.

What is the ideal gas law and what do the variables stand for? (Be sure to include units for all of the variables)

P - (atm, kPa) V - L n - mol R - (L atm/ mol K) or (L kPa / mol K) T - K

What are the units of P, V, and T?

P = atm, mmHg, Torr, atm, kPa V = L T = K

Gay-Lussac's Law

P1/T1=P2/T2

Combined Gas Law

P1V1/T1=P2V2/T2

Boyle's Law

P1V1=P2V2

Ideal Gas Law

PV=nRT

Explain how decreasing the temperature decreases the pressure of a gas using the principles of the kinetic molecular theory.

Pressure is about frequency and force of the collisions of the gas particles with a surface. If you decrease the temperature of a gas, its particles will be moving slower. This causes the frequency of collisions to decrease as it takes the gas longer to get from side to side. It will also decrease the force of the collision.

R (atm)

R = (0.0821 L x atm) / (K x mol)

R (mmHg)

R = (62.4 L x mmHg) / (K x mol)

R (kPa)

R = (8.31 L x kPa) / (K x mol)

Many variables are used to describe gases and some of those variables have multiple units that can be used. When using gas laws some variables must be represented in specific units. Which of the variables below has only one specific unit that can be used when performing calculations using any of the gas laws?

Temperature

The temperature of a sample of matter is the average kinetic energy of the particles. There is a reason it is an average and not the absolute kinetic energy. Thinking back to the motion of the particles in the simulation (look again if needed), what is true of the motion of the individual particles?

The individual gas particles travel at varying individual speeds

Real gases can be liquefied. Explain why, according to the KMT, gases should not be able to be liquefied.

The kinetic theory states that ideal gases have no attractive forces between one another and that the ideal gas particles have no volume of their own. A substance that has no volume cannot be liquifies. If we did - there would be nothing there. Also, liquids stay together due to intermolecular forces of attraction. To turn a liquid into a gas these forces need to be broken - otherwise it would stay a liquid. Turn that around now, for a gas to liquify there would need to be intermolecular forces of attraction forming to hold the liquid together. Since gases are said to not have forces of attraction-they could not be liquified.

Which of the following correctly describes the relationship between the number of collisions and pressure?

The more frequently the gas particles collide with the walls of the container the higher the pressure

What happens to the pressure of a gas as you reach absolute zero? (0K or -273C)

The pressure goes to zero

Explain how such a device can ignite the tinder in the end of the piston using the principles of the kinetic molecular theory.

The quick compression, reduction of volume, will cause the particles to move much faster - heating them up. If done quickly enough the temperature will rise high enough to ignite the tinder.

Consider the relationship between temperature and wall collisions. Which of the following correctly explains the relationship?

There will be more wall collisions at higher temperatures because the particles move faster from one side of the container to the other.

Explain the relationship between the variables pressure and volume?

They are indirectly related, as one goes up, the other must go down. Assuming temperature is constant.

Charles' Law

V1/T1=V2/T2

Why does the value of the ideal gas law constant change depending on what you are given?

We pick the gas law constant based on the units of the measurements we have. If we have a P with unit of atm, we use 0.0821 Latm / molK, if we have a P with unit of kPa, we use 8.314 LkPa / molK. It's about the unit of P.

Real vs. Ideal gases

When describing gases with respect to the kinetic theory we are describing ideal gases. Real gases can be liquified - think about a lighter or your propane tank for your grill. Real gases do have volume - they do take up space. Ideal gases cannot be liquified and do not take up space.

When looking at the relationship between volume and temperature what variable must be held constant? a. Pressure b. Volume c. Moles d. Temperature Explain your answer choice above.

You can only change one variable - have one independent variable - pressure must be held constant. Moles will be held constant by default as you will have a sample of gas = certain number of moles, and then manipulate either the temp or the volume to study relationship.

When looking at the relationship between pressure and temperature what variable must be held constant? a. Pressure b. Volume c. Moles d. Temperature Explain your answer choice above.

You can only change one variable - have one independent variable - volume must be held constant. Moles will be held constant by default as you will have a sample of gas = certain number of moles, and then manipulate either the temp or the pressure to study relationship.

What units are (could be) used for temperature and pressure while studying the relationship between the variables? (Circle all that apply) a. K and Atm b. ̊C and Atm c. K and kPa d. ̊C and kPa

a and c

elastic collision

a collision between ideally elastic bodies in which the final and initial kinetic energies are the same. one in which there is no net loss of total kinetic energy. A collision in which colliding objects rebound without lasting deformation or the generation of heat.

At any given temperature, heavier gas particles will move _____________than the lighter gas particles.

more slowly

Gas particles have completely elastic collisions, meaning they will _______ when they hit one another or the walls of the container.

not lose energy

Explain in your own words Dalton's law of partial pressures.

the total pressure exerted by a mixture of gases equals the sum of the individual pressures of each gas. In other words, if you have a mixture of gases A, B, and C, and A has a P of 2.0atm, B has a P of 1.0atm, and C has a P of 4.0atm, the total pressure exerted by the mixture would be the sum of their pressures, or, 2.0atm + 1.0atm + 4.0atm = 7.0atm.